Many systems have been described for the heterologous expression of nucleic acids encoding e.g. polypeptides such as recombinant proteins in prokaryotic systems. However, most heterologous gene expression systems in prokaryotic host systems have relied exclusively on a limited set of bacterial promoters. The most widely used prokaryotic promoters have included the lactose [lac] (Yanisch-Perron et al., 1985, Gene 33, 103-109), and the tryptophan [trp] (Goeddel et al., 1980, Nature (London) 287, 411-416) promoters, and the hybrid promoters derived from these two [tac and trc] (Brosius, 1984, Gene 27: 161-172; Amann and Brosius, 1985, Gene 40, 183-190). Other inducible promoter systems such as the araB promoter inducible by arabinose (WO 86 04356), the Rhamnose promoter rhaSB inducible by L-rhamnose (WO 03068956) or the rhamnose promoter rhaBAD inducible by L-rhamnose (WO 2004/050877) have been described as well for the heterologous expression of proteins. However, many of the known prokaryotic promoters used for heterologous gene expression have drawbacks such as the toxicity of the heterologous product to the host cell, low rates of expression of the product or the formation of non-functional aggregates (inclusion bodies).
There are further inducible promoter systems which have been used in the homologous expression of proteins such as the melibiose operon inducible by melibiose as described by Belyaeva et al., 2000, Mol. Microbiol. 36(1), 211-222. Most of these inducible promoter systems used for homologous expression have the drawback that only low induction rates and therefore low expression of the desired homologous product have been achieved. Furthermore, these promoter systems are not very tightly regulated thus leading to background activity in an uninduced state, which does not allow for strict expression control. For example Belyaeva et al., used different fragments of the melibiose operon fused to a lacZ gene of E. coli for the expression of β-Galactosidase in E. coli. However, the fragments (KK43, JK19) which produced the highest β-Galactosidase activity produced as well the highest background activity.
Thus, there is a need to provide improved prokaryotic expression systems for the heterologous expression of nucleic acid sequences which have not the above-mentioned to disadvantages.